Optical interference measurements make it possible to measure path length differences between different light paths with great accuracy. Here, light components which have first been guided through different paths are combined. The intensity of the combined light depends on the relative phase of the components which is caused by the path length difference. Thus, by measuring this intensity, information is obtained about the path length difference.
Such measurements are often used in control systems in which the path length difference between the different paths has to be controlled out. When there is no path length difference, the intensity of a combination of the components in which the polarity of one of the components is inverted prior to combining is minimal. This can be used to control out the path length difference. A method of accurately determining the minimum is to use a measuring signal which “crosses zero” when the path length difference is zero (i.e. the measuring signal is negative for a path length difference in one direction and is positive for a path length difference in the opposite direction). Such a measuring signal can be made by slightly modulating the path length difference and synchronously measuring the in-phase component of the resulting modulation in the intensity of the combined signal. This in-phase component crosses zero at the minimum of the intensity, i.e. when the path length difference is zero.
However, in practice, a modulation of the path length difference is often undesired. For that reason, the measuring signal crossing zero, without modulation, is sometimes made by measuring the intensity of two combinations of the components, in which the components are combined directly and after one of the components has been phase-displaced by 180 degrees respectively. The ratio between the difference and the sum of the intensities measured in this manner crosses zero when the path length difference has been controlled out. By calculating this ratio, a measuring signal is obtained by means of which the path length difference can be controlled to zero without modulation being necessary.
The measuring signal obtained in this manner is, inter alia, proportional to the amplitude ratio of the light components which are combined. This amplitude ratio has no effect on the path length difference when it is controlled out. However, the amplitude ratio does play a role when the path length difference is to be controlled to a fixed value which is not equal to zero. Also, the amplitude ratio can affect the gain of a control loop by means of which the path length difference is controlled. Unfortunately, the amplitude ratio of the light components is not always constant. Therefore, it is often necessary to measure the amplitude ratio separately to compensate for its effect. However, this generally requires complicated additional measurements which, moreover, make the apparatus more susceptible to fluctuations in the sensitivity.